The present invention relates to a power supply circuit and a power conversion device for a hybrid car or an electric car.
With increase in the switching speed of a switching device formed as a constituent member of an inverter, the switching loss is reduced, but surge is apt to occur in the collector-emitter voltage of the switching device. In this case, there is a problem that the switching device may be damaged when the voltage is beyond its rated voltage. On the other hand, when the switching speed is slow, surge hardly occurs, but the switching loss of the switching device is large to cause deterioration of the energy efficiency. In addition, when the switching speed is slow, the junction temperature is apt to increase so that the switching device may be damaged when the temperature is beyond its rated temperature.
A gate drive circuit needs to be designed to optimize these trade-offs. It is desirable that a gate voltage does not fluctuate. In order to keep the gate voltage constant, it is necessary to make the output voltage of a power supply circuit not to fluctuate.
In a background-art power supply circuit, with increase in the carrier frequency (switching frequency) of a gate drive circuit, a current with which the capacity in the gate of a switching device is charged/discharged increases to increase power consumption in the gate. On the other hand, a feedback output circuit does not respond to the carrier frequency fc as the gate drive circuit does. As a result, in spite of the increase of power consumption in the power supply circuit, there is no change in an output voltage 601 of the feedback output circuit as represented in FIG. 6. Thus, the electric power supplied to the gate drive circuit is kept constant in spite of the fluctuation of the carrier frequency fc. Even if the power supplied to the gate drive circuit is constant, the power consumption of the gate drive circuit increases. Thus, the power supply voltage drops down as represented in the power supply voltage Vcc of a secondary output circuit in FIG. 6.
As a result, the gate voltage of the switching device is lower than that in the optimized design condition. Thus, there arises a problem that the energy efficiency deteriorates due to the increase in switching loss.
JP-A-2005-341695 discloses an invention of a power supply circuit in which a dummy load circuit provided in an output circuit is turned ON in response to a decrease in a load of the output circuit so as to suppress an increase in an output voltage by increasing the load.
The power supply circuit disclosed in JP-A-2005-341695 indeed takes measures to solve the problem that the output voltage of the output circuit increases when the load of the output circuit decreases, however, there is no consideration for the problem that the output voltage of the output circuit decreases when the load of the output circuit increases.